1. Field of the Invention
The present invention is directed to a circuit structure having at least one MOS transistor in which the MOS transistor has both an improved gate oxide reliability and improved leakage current behavior in conjunction with a low resistance of the gate electrode.
2. Description of the Prior Art
MOS transistors having a short channel length are required for various technical applications, particularly in connection with low voltage/low power applications. Many such MOS transistors have gate electrodes made of heavily doped polysilicon. However, n+-doped silicon and p+-doped silicon differ by 1.1 eV in terms of their work function. As such, in a MOS transistor having a gate electrode made of heavily doped polysilicon, this work function difference results in an increase in the electric field strength between the gate electrode and the drain-side pn junction when the conductivity type of the gate electrode differs from the conductivity type of the source/drain regions. This applies both to MOS transistors having an n+-doped gate electrode and p+-doped source/drain regions and to MOS transistors having a p+-doped gate electrode and n+-doped source/drain regions. This increase in the electric field strength leads to a reduction in the reliability of the gate oxide.
The increase in electric field strength becomes particularly problematic in the case of MOS transistors having channel lengths of less than 0.25 um, such as are used for low voltage/low power applications with supply voltages of less than 3 volts. In such cases, the thickness of the gate oxide is less than 5 nm, since the work function difference is constant and does not scale with the supply voltage.
The leakage current behavior is also impaired as a consequence of this increase in the electric field strength. Indeed, the gate-induced drain leakage current (Gate Induced Drain Leakage GIDL) rises because the band-to-band tunneling probability increases owing to the increased electric field strength.
The increased electric field strength between the gate electrode and the drain-side pn junction can be diminished or removed by using a different material for the gate electrode, this material having a lower or even no work function difference with respect to the material of the source/drain region. It has been proposed to form the gate electrode of a MOS transistor from doped polysilicon which is doped by the same conductivity type as the transistor's source/drain regions (see C. Parrillo, IEDM '85, p. 398). It has also been proposed to form the gate electrode from tin (see, for example, J. M. Hwang et al., IEDM '92, p. 345) or tungsten (see, for example, N. Kasai et al., IEDM '88, p. 242). However, there is a risk of gate oxide damage associated with the use of metal-containing materials in forming gate electrodes. In CMOS circuits, for example, which include both n-channel transistors and p-channel transistors, the use of doped polysilicon of the same conductivity type as the source/drain regions for the gate electrode leads to (1) more complicated process control, (2) the boron penetration effect., (3) lateral dopant diffusion in continuous n+-type/p+-type gate lines, and (4) sensitivity to the gate depletion effect.
Furthermore, it has been proposed to modify the work function of n+-doped or p+-doped polysilicon by means of additional doping with germanium (see, for example, T. J. King et al., IEDM '90, p. 253). This measure, too, leads to complicated process control since, on the one hand, special equipment is required and, on the other hand, it is necessary to adapt the process steps to the altered material properties; for example, during etching.
Most recently, in EP-A-0 657 929, it has been proposed, for the purpose of improving (1) the short-channel effects, (2) the leakage current behavior and (3) the reliability of the gate oxide, that the n+-doped gate electrode be counter-doped by means of additional boron implantation in the case of p-channel MOS transistors having an n+-doped gate electrode. Such counter-doping alters the work function of the gate electrode without altering its conductivity type. The dopant concentration in the gate electrode is changed as a result of this measure. Accordingly, the resistance of the gate electrode rises, which then leads to a voltage drop across the gate electrode.